(1) Field of the Invention
The present invention relates to an improved starting device for starting a high pressure metal vapor discharge lamp, particularly a metal halide lamp.
(2) Description of the Prior Art
A metal halide lamp or high pressure sodium lamp cannot be started by a 200 V commercial power source with a ballast for a conventional mercury lamp because the starting voltage of the metal halide lamp or high pressure sodium lamp is much higher than that of the mercury lamp. Accordingly, improvements have been made on the lamp per se or starting devices, and at the present, a high pressure metal vapor discharge lamp of the low starting voltage type, which can be started by the ballast for the mercury lamp, is put into practical use.
An instance of the starting device enabling starting of a high pressure metal vapor discharge lamp by a 200 V commercial power source with a ballast for a conventional mercury lamp is illustrated in FIG. 1. As is seen from FIG. 1, in this starting device, a starting aid circuit 12 comprising, connected in series, an anti-snap thermal switch 4 which is closed at room temperature and opened at a high temperature, a heater 5 for opening the thermal switch 4 and a resistor 6 for controlling an electric current flowing in the heater 5 is connected to an arc tube 3 in parallel thereto and communicated with a 200 V commercial power source 1 through an inductive ballast 2 for a mercury lamp. The starting aid circuit 12 and arc tube 3 are arranged in an outer envelope 7. When a power is applied from the power source 1, an electric current flows in the starting aid circuit 12 and the heater 5 generates the heat. The thermal switch 4 heated by the heater 5 is opened to cut the electric current flowing in the heater 5. Accordingly, the thermal switch 4 is cooled and is then closed again. As is well known, when this thermal switch 4 is opened, a single high voltage pulse is generated between both the terminals of the inductive ballast 2. However, a high pressure metal vapor discharge lamp, especially a metal halide lamp, can hardly be started by this single high voltage pulse. When the thermal switch 5 is cooled and closed, a great number of high frequency pulses are overlapped to the power source voltage by the chattering phenomenon of the thermal switch 4, as shown in FIG. 3, and this state is retained during a considerable period. The time t shown in FIG. 3 indicates the duration time of such high frequency pulses, and this time t is ordinarily in the range of from 0.01 second to several seconds. By this duration of high frequency pulses, the metal halide lamp can be started at a relatively low pulse voltage (V.sub.p).
FIGS. 2-A and 2-B illustrate one embodiment of the structure of the thermal switch 4 in FIG. 4. FIG. 2-A is a front view and FIG. 2-B is a side view. The thermal switch 4 comprises contact rods 8 and 8' composed of a high-melting-point metal, a bimetal plate 9, supporters 10 and 10' and an insulator 11 for fixing the supporters 10 and 10'. Pure tungsten rods are used as the contact rods 8 and 8', and the supporters 10 and 10' and the insulator 11 comprise, for example, a combination of Mo wires and Mo glass or a combination of Kovar and Kovar glass.
The most important characteristics of this starting device are the time t of duration of high frequency pulses (see FIG. 3) and the amplitude V.sub.p of high frequency pulses (see FIG. 3), and the reliability of the starting performance is influenced by values of these characteristics. More specifically, the longer is the duration time t or the larger is the high frequency pulse amplitude V.sub.p, the more improved is the reliability of the starting performance of the starting device. However, if the high frequency pulse amplitude V.sub.p is too large, reduction of the insulating ability of the ballast, a screw base or other lighting equipment is accelerated. Accordingly, it is necessary to maintain the high frequency pulse amplitude V.sub.p at a level not exceeding 5 KV. Therefore, it has been desired to prolong the duration time t of high frequency pulses assuredly by some means or other in order to improve the starting characteristic.